It is known from U.S. Pat. No. 7,813,599 B2 to equip an optical waveguide with three cores. Each of these cores shall have a plurality of Bragg gratings. Part of the light coupled into the cores is reflected on the fiber gratings and part thereof is transmitted. The reflected wavelength and/or the reflected wavelength range depend on the grating constants of the Bragg grating.
The grating constant, in turn, is determined when the Bragg grating is produced and, after the production thereof, the constant is changed under the influence of mechanical tension or temperature. If the optical waveguide has a curvature, the three cores include different radii of curvature, and therefore each core has a different mechanical tension. For example, a core which is closer to the curvature center is compressed and a core which is farther away from the curvature center is stretched. As a result, the form of the optical waveguide in space can be determined by determining the wavelengths reflected by the Bragg gratings.
However, this known sensor has the drawback that it requires a costly optical system to couple light in and out of three cores. Since the three cores have to be read out individually, the demands made on the spectrometric device of the respectively reflected wavelengths are increased. If the form of the optical waveguide changes with time, the measurement of the reflected spectra from all cores also has to be synchronized.